1. Field of the Invention
The present invention relates to a transfer device for use with standardized mechanical interface (SMIF) systems for facilitating semiconductor wafer fabrication, and in particular to a transfer mechanism for gripping and transport of a semiconductor wafer cassette along a vertical axis.
2. Description of the Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 .mu.m to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half micron (.mu.m) and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.2 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) sealed pods, having a minimal volume, used for storing and transporting cassettes which hold the semiconductor wafers; (2) enclosures placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and enclosures (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer and/or wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.
It is important that a precisely controllable system be provided for transferring wafer cassettes from, for example, a SMIF pod to within a semiconductor wafer processing station in order to avoid damaging the wafers within the cassette. Any such damage may be significant as a single cassette may presently carry as much as $20,000 to $30,000 worth of 200 mm wafers.
There are presently several known schemes for transferring a semiconductor wafer cassette into a wafer processing station. Two common schemes are shown in FIGS. 1A and 1B, respectively. In the system of FIG. 1A, after a semiconductor wafer cassette 20 has been separated from a SMIF pod (not shown), the cassette may be loaded into a cassette chamber 22 of a wafer processing station 24 by first vertically raising or lowering the cassette along a Y-axis to the properly align the cassette with the cassette chamber, and second by horizontally moving the cassette along an X-axis into the cassette chamber.
The system of FIG. 1B differs from the system of FIG. 1A in that the processing station 24 includes a platform 26 that extends out of the cassette chamber 22 when a cassette is to be loaded into the chamber. In some processing stations, the wafer cassette is desirably loaded deep within the cassette chamber. In such instances, the wafer cassette transfer device may load the cassette on to the extended platform, which platform may then be retracted into the cassette chamber with the cassette supported thereon. In the system of FIG. 1B, after the cassette is positioned adjacent to the processing station 24, the cassette 20 need only be moved vertically along the Y-axis until the cassette is located above the platform, and then lowered along the Y-axis onto the platform after the platform has been extended. Thus, the system of FIG. 1A requires a transfer mechanism capable of moving in both the X and Y directions, whereas the system of FIG. 1B requires a transfer mechanism capable of moving solely in the Y direction.
A conventional transfer mechanism for locating a cassette within a processing station cassette chamber for the systems of both FIGS. 1A and 1B is shown in FIG. 1C. A SMIF pod, including a top 34 and a door 36 for supporting the wafer cassette 20, is positioned on an indexer 38. The indexer first decouples the pod top from the pod door, and then lowers the pod door with the cassette thereon adjacent to the cassette chamber 22 of the processing station 24. Thereafter, a pivotally mounted robotic transfer arm 40 transfers the cassette 20 from the pod door 36 on the indexer to a cassette platform 42 (FIG. 1C) within the chamber 22, or an extended platform 26 (FIG. 1B) outside of the chamber 22. A top gripping mechanism 44 is provided on the free end of the arm 40 for gripping a features conventionally provided on a top surface of the cassette 20.
For a processing station such as that shown in FIGS. 1A and 1C, a transfer mechanism such as described above capable of transporting a cassette in both the X and Y directions is necessary in order to load a cassette into the cassette chamber. However, where movement of the cassette is required only in the Y direction, such as where a processing station includes an extendable platform as shown in FIG. 1B, a more simple and space efficient design may be accomplished.